1. Field of the Invention
The present invention relates to a magneto-optical recording medium on/from which information is recorded/reproduced by a laser beam by utilizing a magneto-optical effect, and more particularly, to a magneto-optical recording medium which can realize high-density recording, and a magneto-optical reproduction method.
2. Related Background Art
As a rewritable high-density recording method, a magneto-optical recording medium on which information is recorded by writing magnetic domains on a magnetic thin film using heat energy of a semiconductor laser, and from which the information is read out using the magneto-optical effect has received a lot of attention. In recent years, demand has arisen for developing a recording medium of a larger capacity by increasing the recording speed of the magneto-optical recording medium.
The line recording density of an optical disk such as a magneto-optical recording medium largely depends on the laser wavelength of a reproduction optical system and the numerical aperture of an objective lens. More specifically, when the wavelength .lambda. of the reproduction optical system and the numerical aperture NA of the objective lens are determined, the diameter of the beam waist is determined. For this reason, the lower limit of a mark period which can be reproduced and detected is about .lambda./2NA.
On the other hand, the track density is mainly limited by crosstalk. The crosstalk is mainly determined by the distribution (profile) of a laser beam on the medium surface, and is expressed as a function of .lambda./2NA like the mark period.
Therefore, in order to develop a high density conventional optical disk, the wavelength of a laser of the reproduction optical system must be shortened, and the numerical aperture NA of the objective lens must be increased. However, it is not easy to shorten the wavelength of the laser due to problems associated with the efficiency, heat generation, and the like of the elements thereof. On the other hand, when the numerical aperture of the objective lens is increased, the distance between the lens and the disk becomes too small, and a mechanical problem such as a collision may occur. For these reasons, a technique for increasing the recording density by modifying the arrangement of the recording medium or the reading method has been developed.
For example, in Japanese Laid-Open Patent Application Nos. 3-93058 and 4-255946, attempts have been made to improve the recording density by magnetic super-resolution using a medium comprising a reproducing layer and a recording layer. In this technique, an initialization external magnetic field is applied in advance to a medium, which basically comprises a reproducing layer and a recording layer and also comprises an auxiliary layer and an intermediate layer for the purpose of improving the medium characteristics, by aligning the direction of magnetization of the reproducing layer in one direction before reproduction of a signal. Thereafter, magnetic domain information on the recording layer is masked, and a light spot irradiates the medium. Of temperature distributions formed on the medium in this case, magnetic domain information on the recording layer can be transferred to and reproduced by only a reproducing layer portion in a high-temperature region. In this manner, intersymbol interference upon reproduction is decreased, and a signal having a period equal to or smaller than the diffraction limit of light can be reproduced, thereby improving the recording density.
However, since the super-resolution method described in Japanese Laid-Open Patent Application No. 3-93058 requires a large initialization magnet and a reproducing magnetic field for reproduction, the drive device becomes expensive, and a compact structure is difficult to achieve.
In order to solve these problems, the present inventor examined a magneto-optical recording medium which can realize magnetic super-resolution without applying a reproducing magnetic field, and a reproducing method of the magneto-optical recording medium. As shown in FIG. 1, using a medium having a two-layered structure in which a magnetic layer (reproducing layer) whose magnetization is oriented in the in-plane direction at room temperature, and is oriented in the perpendicular direction when the temperature rises, and a magnetic layer (recording layer) having perpendicular magnetic anisotropy are stacked, magnetization information on the recording layer is transferred to only a high-temperature portion, which is irradiated with a light spot and becomes a perpendicular magnetization film, of the reproducing layer upon reproduction, thereby realizing magnetic super-resolution without applying any reproducing magnetic field. This method does not require an operation for aligning the direction of magnetization of the reproducing layer in one direction in advance, and the like, and can reproduce a signal having a period equal to or smaller than the diffraction limit of light.
However, in such a two-layered super-resolution magneto-optical recording medium using an in-plane magnetization film, when in-plane anisotropy is increased at room temperature, magnetization information on the recording layer can be sufficiently masked, but it is difficult to convert this film into a perfect perpendicular magnetization film at the reproducing temperature. For example, when an RE (rare earth) rich heavy rare earth-iron group transition metal alloy is used in the reproducing layer, if the Co addition amount is increased not to decrease the Curie temperature and the content of a rare earth element is increased to increase Ms at room temperature and to increase in-plane anisotropy, the compensation temperature increases accordingly, and Ms cannot become sufficiently small at the reproducing temperature. As a result, a perfect perpendicular magnetization film cannot be obtained at the reproducing temperature.
On the contrary, when in-plane anisotropy at room temperature is decreased, a perfect perpendicular magnetization film can be obtained at the reproducing temperature, but an interface magnetic wall formed between the reproducing layer and the recording layer at room temperature is mainly formed at the reproducing layer side, as shown in FIG. 2. More specifically, a magnetization component in the perpendicular direction conforming to the magnetization information on the recording layer is formed in a portion, near the recording layer, of the reproducing layer. Therefore, it is difficult to perfectly mask the magnetization information on the recording layer by the reproducing layer.
Therefore, in the above-mentioned two-layered super-resolution magneto-optical recording medium using an in-plane magnetization film, when the recording mark length or track width is shortened, it is not easy to obtain a satisfactory reproducing signal.